Filler-reinforced polyamides are extensively used as industrial materials because of their high strength and rigidity. However, reinforced polyamides in which the resin consists of one or more crystalline polyamides alone have insufficient weld strength and, hence, there are cases where applications thereof are limited. This is because molded parts thereof having many welds or molded parts or products thereof in which the welds are required to have a high safety factor have problems, for example, that they break or crack at welds or suffer considerable deterioration at welds in a high-temperature or high-humidity atmosphere or under high load.
On the other hand, in the case of reinforced polyamides in which the resin consists of one or more noncrystalline polyamides alone, molded objects obtained therefrom have sufficient weld strength. However, applications of these reinforced polyamides also are limited because they require a long molding time.
A known technique for eliminating the above problems is to improve weld strength by using a copolymer having a low melting point, such as, e.g., polyamide 6/66. However, this technique has a drawback that the composition not only has insufficient absolute strength but is reduced in heat and chemical resistance.
JP-A-5-112672 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes a technique for weld strength improvement which comprises adding a specific acicular filler to a thermoplastic resin such as a polyamide. However, molded parts of this composition have insufficient strength in parts other than welds, i.e., have insufficient base material strength, although the weld strength is improved in some degree. This background art composition is hence unusable as structural parts required to have high base material strength.
JP-A-7-149947 proposes a technique for weld strength improvement which comprises adding a silicone rubber to a thermoplastic resin reinforced with a fibrous filler. However, this composition is reduced in rigidity and heat resistance although improved in weld strength.
JP-A-55-62959 and JP-A-6-172643 each proposes a technique which comprises adding an aromaticpolyamide. However, the technique disclosed in JP-A-55-62959 necessitates the addition of a large amount of an aromatic polyamide and is hence unable to give an inexpensive composition. In addition, this background art is intended to improve surface gloss and there is no description therein to the effect that weld strength is improved. The technique disclosed in JP-A-6-172643 is insufficient in weld strength improvement because an aromatic dicarboxylic acid is used as crystallinity-imparting monomer in producing the aromatic polyamide.